Journal Articles
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Item Intragastric restructuring dictates the digestive kinetics of heat-set milk protein gels of contrasting textures(Elsevier, 2024-11) Li S; Mungure T; Ye A; Loveday SM; Ellis A; Weeks M; Singh HThe gelation of milk proteins can be achieved by various means, enabling the development of diverse products. In this study, heat-set milk protein gels (15 % protein) of diverse textures were made by pH modulation and two gels were selected for dynamic in vitro gastric digestion: a spoonable soft gel (SG, pH 6.55' G' of ∼100 Pa) and a sliceable firm gel (FG, pH 5.65; G' of ∼7000 Pa). The two gels displayed markedly different structural changes and digestion kinetics during gastric digestion. The SG underwent substantial structural compaction during the first 120 min of gastric digestion into a denser and firmer gastric chyme (26.3 % crude protein, G* of ∼8500 Pa) than the chyme of the FG (15.7 % crude protein, G* of ∼3000 Pa). These contrasting intragastric structural changes of the gels reversed their original textural differences, which led to slower digestion and gastric emptying of proteins from the SG compared with the FG. The different intragastric pH profiles during the digestion of the two gels likely played a key role by modulating the proteolytic activity and specificity (to κ-casein) of pepsin. Preferential early cleavage of κ-casein in SG stimulated coagulation and compaction of solid chyme, whereas rapid hydrolysis of αS- and β-caseins in the FG weakened coagulation. This study provided new insights into controlling the structural development of dairy-based foods during gastric digestion and modulating digestion kinetics.Item Kinetics of heat-induced interactions among whey proteins and casein micelles in sheep skim milk and aggregation of the casein micelles(Elsevier Inc on behalf of the American Dairy Science Association, 2022-05) Pan Z; Ye A; Dave A; Fraser K; Singh HThe interactions among the proteins in sheep skim milk (SSM) during heat treatments (67.5-90°C for 0.5-30 min) were characterized by the kinetics of the denaturation of the whey proteins and of the association of the denatured whey proteins with casein micelles, and changes in the size and structure of casein micelles. The relationship between the size of the casein micelles and the association of whey proteins with the casein micelles is discussed. The level of denaturation and association with the casein micelles for β-lactoglobulin (β-LG) and α-lactalbumin (α-LA) increased with increasing heating temperature and time; the rates of denaturation and association with the casein micelles were markedly higher for β-LG than for α-LA in the temperature range 80 to 90°C; the Arrhenius critical temperature was 80°C for the denaturation of both β-LG and α-LA. The casein micelle size increased by 7 to 120 nm, depending on the heating temperature and the holding time. For instance, the micelle size (about 293 nm) of SSM heated at 90°C for 30 min increased by about 70% compared with that (about 174.6 nm) of unheated SSM. The casein micelle size increased slowly by a maximum of about 65 nm until the level of association of the denatured whey proteins with casein micelles reached 95%, and then increased markedly by a maximum of about 120 nm when the association level was greater than about 95%. The marked increases in casein micelle size in heated SSM were due to aggregation of the casein micelles. Aggregation of the casein micelles and association of whey protein with the micelles occurred simultaneously in SSM during heating.Item pH-dependent sedimentation and protein interactions in ultra-high-temperature-treated sheep skim milk(Elsevier Inc and Fass Inc on behalf of the American Dairy Science Association, 2023-03) Pan Z; Ye A; Dave A; Fraser K; Singh HSheep milk is considered unstable to UHT processing, but the instability mechanism has not been investigated. This study assessed the effect of UHT treatment (140°C/5 s) and milk pH values from 6.6 to 7.0 on the physical properties of sheep skim milk (SSM), including heat coagulation time, particle size, sedimentation, ionic calcium level, and changes in protein composition. Significant amounts of sediment were found in UHT-treated SSM at the natural pH (∼6.6) and pH 7.0, whereas lower amounts of sediment were observed at pH values of 6.7 to 6.9. The proteins in the sediment were mainly κ-casein (CN)-depleted casein micelles with low levels of whey proteins regardless of the pH. Both the pH and the ionic calcium level of the SSM at all pH values decreased after UHT treatment. The dissociation levels of κ-, β-, and αS2-CN increased with increasing pH of the SSM before and after heating. The protein content, ionic calcium level, and dissociation level of κ-CN were higher in the SSM than values reported previously in cow skim milk. These differences may contribute to the high amounts of sediment in the UHT-treated SSM at natural pH (∼6.6). Significantly higher levels of κ-, β-, and αS2-CN were detected in the serum phase after heating the SSM at pH 7.0, suggesting that less κ-CN was attached to the casein micelles and that more internal structures of the casein micelles may have been exposed during heating. This could, in turn, have destabilized the casein micelles, resulting in the formation of protein aggregates and high amounts of sediment after UHT treatment of the SSM at pH 7.0.Item Acid and rennet gelation properties of sheep, goat, and cow milks: Effects of processing and seasonal variation(Elsevier Inc on behalf of the American Dairy Science Association, 2023-03) Li S; Delger M; Dave A; Singh H; Ye AGelation is an important functional property of milk that enables the manufacture of various dairy products. This study investigated the acid (with glucono-δ-lactone) and rennet gelation properties of differently processed sheep, goat, and cow milks using small-amplitude oscillatory rheological tests. The impacts of ruminant species, milk processing (homogenization and heat treatments), seasonality, and their interactions were studied. Acid gelation properties were improved (higher gelation pH, shorter gelation time, and higher storage modulus (G') by intense heat treatment (95°C for 5 min) to comparable extents for sheep and cow milks, both better than those for goat milk. Goat milk produced weak acid gels with low G' (<100 Pa) despite improvements induced by heat treatments. Seasonality had a marked impact on the acid gelation properties of sheep milk. The acid gels of late-season sheep milk had a lower gelation pH, no maximum in tan δ following gel formation, and 70% lower G' values than those from other seasons. We propose the potential key role of a critical acid gelation pH that induces structural rearrangements in determining the viscoelastic properties of the final gels. For rennet-induced gelation, compared with cow milk, the processing treatments of the goat and sheep milks had much smaller impacts on their gelation properties. Intense heat treatment (95°C for 5 min) prolonged the rennet gelation time of homogenized cow milk by 8.6 min (74% increase) and reduced the G' of the rennet gels by 81 Pa (85% decrease). For sheep and goat milks, the same treatment altered the rennet gelation time by only less than 3 min and the G' of the rennet gels by less than 14 Pa. This difference may have been caused by the different physicochemical properties of the milks, such as differences in their colloidal stability, proportion of serum-phase caseins, and ionic calcium concentration. The seasonal variations in the gelation properties (both acid and rennet induced) of goat milk could be explained by the minor variation in its protein and fat contents. This study provides new perspectives and understandings of milk gelation by demonstrating the interactive effects among ruminant species, processing, and seasonality.Item The Effect of pH and Sodium Caseinate on the Aqueous Solubility, Stability, and Crystallinity of Rutin towards Concentrated Colloidally Stable Particles for the Incorporation into Functional Foods(MDPI (Basel, Switzerland), 2022-01-14) Rashidinejad A; Jameson GB; Singh H; Papetti APoor water solubility and low bioavailability of hydrophobic flavonoids such as rutin remain as substantial challenges to their oral delivery via functional foods. In this study, the effect of pH and the addition of a protein (sodium caseinate; NaCas) on the aqueous solubility and stability of rutin was studied, from which an efficient delivery system for the incorporation of rutin into functional food products was developed. The aqueous solubility, chemical stability, crystallinity, and morphology of rutin (0.1-5% w/v) under various pH (1-11) and protein concentrations (0.2-8% w/v) were studied. To manufacture the concentrated colloidally stable rutin-NaCas particles, rutin was dissolved and deprotonated in a NaCas solution at alkaline pH before its subsequent neutralisation at pH 7. The excess water was removed using ultrafiltration to improve the loading capacity. Rutin showed the highest solubility at pH 11, while the addition of NaCas resulted in the improvement of both solubility and chemical stability. Critically, to achieve particles with colloidal stability, the NaCas:rutin ratio (w/w) had to be greater than 2.5 and 40 respectively for the lowest (0.2% w/v) and highest (4 to 8% w/v) concentrations of NaCas. The rutin-NaCas particles in the concentrated formulations were physically stable, with a size in the range of 185 to 230 nm and zeta potential of -36.8 to -38.1 mV, depending on the NaCas:rutin ratio. Encapsulation efficiency and loading capacity of rutin in different systems were 76% to 83% and 2% to 22%, respectively. The concentrated formulation containing 5% w/v NaCas and 2% w/v rutin was chosen as the most efficient delivery system due to the ideal protein:flavonoid ratio (2.5:1), which resulted in the highest loading capacity (22%). Taken together, the findings show that the delivery system developed in this study can be a promising method for the incorporation of a high concentration of hydrophobic flavonoids such as rutin into functional foods.Item Kinetics of pepsin-induced hydrolysis and the coagulation of milk proteins(Elsevier Inc and the Federation of Animal Science Societies on behalf of the American Dairy Science Association, 2022-02) Yang M; Ye A; Yang Z; Everett DW; Gilbert EP; Singh HHydrolysis-induced coagulation of casein micelles by pepsin occurs during the digestion of milk. In this study, the effect of pH (6.7–5.3) and pepsin concentration (0.110–2.75 U/mL) on the hydrolysis of κ-casein and the coagulation of the casein micelles in bovine skim milk was investigated at 37°C using reverse-phase HPLC, oscillatory rheology, and confocal laser scanning microscopy. The hydrolysis of κ-casein followed a combined kinetic model of first-order hydrolysis and putative pepsin denaturation. The hydrolysis rate increased with increasing pepsin concentration at a given pH, was pH dependent, and reached a maximum at pH ~6.0. Both the increase in pepsin concentration and decrease in pH resulted in a shorter coagulation time. The extent of κ-casein hydrolysis required for coagulation was independent of the pepsin concentration at a given pH and, because of the lower electrostatic repulsion between para-casein micelles at lower pH, decreased markedly from ~73% to ~33% when pH decreased from 6.3 to 5.3. In addition, the rheological properties and the microstructures of the coagulum were markedly affected by the pH and the pepsin concentration. The knowledge obtained from this study provides further understanding on the mechanism of milk coagulation, occurring at the initial stage of transiting into gastric conditions with high pH and low pepsin concentration.